Asked by Melissa
The 13th termof an arithmetic sequence is 61 and the 20th term of the squence is 82. Calculate the sum of the first 25 terms of the sequence
Answers
Answered by
Steve
T13 = a+12d = 61
T20 = a+19d = 82
so,
T20-T13 = 7d = 21
d = 3
a = 25
S25 = 25/2 (25 + 25+24*3)
= 25/2 (122) = 25*61 = 1525
T20 = a+19d = 82
so,
T20-T13 = 7d = 21
d = 3
a = 25
S25 = 25/2 (25 + 25+24*3)
= 25/2 (122) = 25*61 = 1525
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